The present invention relates to a magnetic disk drive apparatus. More particularly, the invention relates to a diagnostic circuit for setting conditions of a magnetic recording system (channel), such as an equalizer, and a magnetic disk drive apparatus which automatically optimizes a signal processing system.
FIG. 2 shows one example of recording and reproducing channels in a magnetic disk drive apparatus. Information to be recorded is converted into a proper code format, such as NRZ-I code, suitable for magnetic recording by an encoder 18. Subsequently, the encoded information is fed to a write amplifier 20 via a recording compensation circuit 19. The write amplifier 20 then converts the information into a recording current waveform. The recording current is converted into a recording magnetic field by means of a recording magnetic head 21 for causing or not causing magnetization reversal on a recording medium 22 and thus recording information respectively corresponding to "1" or "0". In the reproducing process, the magnetic head 21 detects magnetic flux leaking from the magnetization reversal portion on the recording medium, i.e., magnetic disk to convert it into an electric signal. In case of an induction type magnetic head, peaks appear in the converted electric signal corresponding to recorded codes "1". This signal is fed to a pulse generation circuit 27 through an amplifier 23, an automatic gain controlled (AGC) amplifier 24, a waveform equalizer 25 and a differentiation circuit 26. The pulse generation circuit 27 converts the signal thus received into a peak pulse on a line 28. The peak pulse is compared with a VCO clock on a line 30 from a VFO circuit 29 by a detector 31 and thus converted into a binary signal. Then, the binary signal is decoded by a decoder 32 into an original information format.
In such a magnetic recording channel, the recording compensation circuit 19 and the equalizer 25 are provided for compensating nonlineality and high-frequency attenuation which can be caused during recording and reproducing processes. For example, when the NRZ-I code as shown in FIG. 3 is recorded and reproduced, at the portion where a distance between the magnetization reversals is small, the position of the magnetization reversal is shifted forward on the recording medium due to the effect of self-demagnetization and recording demagnetization. Therefore, the reproduced waveform is shifted toward the left in FIG. 3. Furthermore, at the portion where the distance between the magnetization reversals is small, the waves reproduced from the portions of the magnetization reversal overlap with adjacent reproduced waveforms to cause shifting of the peak positions. Such shifting of peaks tends to lead to errors in detection. Therefore, the recording compensation circuit 19 provisionally shifts the reversal position of the recording code for canceling the shift of the peak position. On the other hand, the equalizer 25 makes the reproduced waveform thinner for avoiding overlapping of the adjacent reproduced waveforms even at the portion where the distance of magnetization reversals is small.
Since the modern magnetic disk drive apparatus has extremely high recording density, signal processing technologies, such as equalizers, are essential. Also, it is important to use the equalizer and so forth adapting to the characteristics of the magnetic disk drive apparatus. Therefore, the trend is to move from a cosine-equalizer as shown in FIG. 4 (see Nishiyama et al., U.S. Pat. No. 4,644,424, issued Feb. 17, 1987, commonly owned), to a high performance transversal-equalizer as shown in FIG. 5 (see Ouchi et al., U.S. Pat. No. 4,651,236, issued Mar. 17, 1987, commonly owned).